Visibility modifiers (public, private, internal, external) in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the function accessible from anywhere.

Blockchain / Solidity

function greet() [1] returns (string memory) {
    return "Hello!";
}

Drag options to blanks, or click blank then click option'

Aprivate

Bpublic

Cinternal

Dexternal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'private' makes the function inaccessible from outside the contract.

Using 'internal' restricts access to the contract and its children only.

The public visibility modifier allows the function to be called from anywhere, both inside and outside the contract.

2fill in blank

medium

Complete the code to make the variable accessible only within the contract and derived contracts.

Blockchain / Solidity

uint256 [1] myNumber;

Drag options to blanks, or click blank then click option'

Ainternal

Bprivate

Cpublic

Dexternal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' exposes the variable to everyone.

Using 'private' restricts access only to the contract itself.

The internal modifier restricts access to the contract itself and any contracts that inherit from it.

3fill in blank

hard

Fix the error in the function declaration to allow it to be called only from outside the contract.

Blockchain / Solidity

function externalCall() [1] {
    // function logic
}

Drag options to blanks, or click blank then click option'

Aexternal

Bprivate

Cpublic

Dinternal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' allows both internal and external calls.

Using 'private' restricts calls to inside the contract only.

The external modifier means the function can only be called from outside the contract, not from inside.

4fill in blank

hard

Fill both blanks to create a private variable and a public function to access it.

Blockchain / Solidity

uint256 [1] secretNumber;
function getSecret() [2] view returns (uint256) {
    return secretNumber;
}

Drag options to blanks, or click blank then click option'

Aprivate

Bpublic

Cinternal

Dexternal

Attempts:

3 left

💡 Hint

Common Mistakes

Making the variable public exposes it directly.

Making the function private or internal hides it from external callers.

The variable is private so it can't be accessed directly outside the contract. The function is public so anyone can call it to get the value.

5fill in blank

hard

Fill all three blanks to define an internal variable, a private function, and a public function that calls the private one.

Blockchain / Solidity

uint256 [1] data;


function calculate() [2] view returns (uint256) {
    return data * 2;
}


function getResult() [3] view returns (uint256) {
    return calculate();
}

Drag options to blanks, or click blank then click option'

Ainternal

Bprivate

Cpublic

Dexternal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' for the private function exposes it.

Using 'private' for the public function hides it from outside.

The variable is internal so accessible inside the contract and children. The private function can only be called inside this contract. The public function allows anyone to call it and get the result.

Practice

1. Which visibility modifier allows a function to be called both inside and outside the contract, including by other contracts and users?

easy

A. public

B. private

C. internal

D. external

Solution

1. Step 1: Understand visibility modifiers

   Public functions can be accessed anywhere, inside or outside the contract.

2. Step 2: Compare with other modifiers

   Private is only inside the contract, internal is inside and inherited contracts, external only callable from outside.

3. Final Answer:

   public -> Option A

4. Quick Check:

   public = open access [OK]

Hint: Public means open to all inside and outside calls [OK]

Common Mistakes:

• Confusing private with public access
• Thinking internal allows outside calls
• Assuming external works inside contract

2. Which of the following is the correct way to declare a function that can only be called from outside the contract in Solidity?

easy

A. function foo() private {}

B. function foo() external {}

C. function foo() internal {}

D. function foo() public {}

Solution

1. Step 1: Identify external function syntax

   External functions are declared with the keyword external after the function name.

2. Step 2: Check other options

   Private and internal restrict access inside the contract or inheritance, public allows all calls.

3. Final Answer:

   function foo() external {} -> Option B

4. Quick Check:

   external = outside calls only [OK]

Hint: External functions use keyword 'external' after function name [OK]

Common Mistakes:

• Using private instead of external
• Confusing public with external
• Missing parentheses or braces

3. Consider the following Solidity contract snippet:

contract Test {
    uint private data = 10;
    function getData() public view returns (uint) {
        return data;
    }
    function setData(uint _data) external {
        data = _data;
    }
}

What will happen if another contract tries to call data directly?

medium

A. It will succeed and return 10

B. It will fail because data is external

C. It will fail because data is private

D. It will succeed because data is public by default

Solution

1. Step 1: Check variable visibility

   The variable data is declared private, so it cannot be accessed outside the contract.

2. Step 2: Understand access from other contracts

   Other contracts cannot access private variables directly, only public or external functions.

3. Final Answer:

   It will fail because data is private -> Option C

4. Quick Check:

   private = no outside access [OK]

Hint: Private variables can't be accessed outside their contract [OK]

Common Mistakes:

• Assuming private variables are public by default
• Confusing external with private
• Thinking public functions expose private variables directly

4. Given this Solidity code snippet:

contract Sample {
    uint internal count = 0;
    function increment() private {
        count += 1;
    }
    function callIncrement() public {
        increment();
    }
}

Which of the following fixes the error preventing increment() from being called?

medium

A. No fix needed, code is correct

B. Change increment() visibility to internal

C. Change increment() visibility to external

D. Change increment() visibility to public

Solution

1. Step 1: Check function call context

   The function increment() is private and called inside the same contract by callIncrement().

2. Step 2: Understand private function access

   Private functions can be called inside the same contract, so this call is valid.

3. Final Answer:

   No fix needed, code is correct -> Option A

4. Quick Check:

   private functions callable inside contract [OK]

Hint: Private functions work inside same contract only [OK]

Common Mistakes:

• Thinking private functions can't be called inside contract
• Confusing internal with private
• Changing visibility unnecessarily

5. You want to create a Solidity contract where a function can only be called by this contract or contracts that inherit from it, but not by external users. Which visibility modifier should you use for this function?

hard

A. public

B. external

C. private

D. internal

Solution

1. Step 1: Understand access needs

   The function should be callable inside this contract and by derived contracts, but not externally.

2. Step 2: Match visibility modifier

   Internal visibility allows access inside the contract and by inheriting contracts only.

3. Final Answer:

   internal -> Option D

4. Quick Check:

   internal = contract + inheritance only [OK]

Hint: Internal means accessible inside and by child contracts only [OK]

Common Mistakes:

• Choosing public which allows external calls
• Choosing private which blocks inheritance access
• Confusing external with internal